Build fails with Qt >= 6.10

A change in Qt internal #includes appears to have made `QElapsedTimer` no longer visible in `sacneffectengine.h`:

```
/home/dankeenan/Documents/Projects/sacnview/src/sacn/sacneffectengine.h:124:5: error: ‘QElapsedTimer’ does not name a type
  124 |     QElapsedTimer m_dwellTimeTracker;
      |     ^~~~~~~~~~~~~
```